Annual Desert Crops Workshop covers season's problem areas

Six new fusarium wilt sites have been identified so far in the desert lettuce in the Yuma area this season, bring the total to 23 locations where the disease has been discovered since first identified two years ago.

That is not good news since there is no cure for the disease that causes lettuce rot, University of Arizona extension plant pathologist Mike Matheron told the 14th annual Desert Crops Workshop in Holtville, Calif., earlier this month.

“The good news is that 99 percent of the lettuce fields in Yuma are fusarium wilt free,” said Matheron.

However, Matheron said that number could fall dramatically if producers are not careful.

“Once you have the disease, you cannot get rid of it,” he said. It kills seedling lettuce and can render older plants unmarketable.

The simplest way to avoid it in infected fields is to plant another lettuce crop on the field,” The fusarium wilt found in Yuma lettuce does not impact any other crop, including other vegetables.

Where wilt has been identified, Matheron said producers will be diligent in cleaning equipment before moving to another field. However, he is fearful that when another crop is planted in that infected field, growers may not be so diligent.

“Every effort should be made to prevent the spread of contaminated soil from known locations of fusarium to ‘clean’ fields by workers and equipment. These precautions should be maintained even when crops other than lettuce are grown in infested fields,” warned Matheron.

If a grower must plant lettuce on an infected field, Matheron said he could plant Romaine, which is more tolerant of fusarium than other lettuce types.

Or, a grower can plant later.

Lost test varieties

All September-planted lettuce varieties were wiped out in Matheron's test because weather conditions are ideal for the disease to develop then. None of the cultivars planted then were even mildly tolerant.

Fusarium was moderate in a second planting (Oct 17) and very mild in the third planting (Dec. 6).

In some lettuce cultivars in the second planting and most cultivars in the third planting survived.

The first discovery of fusarium wilt of lettuce in the U.S. was during 1990 in Fresno County near Huron, Calif.

“Fusarium wilt presents a serious threat to the health of the lettuce industry in Arizona,” Matheron says.

Development of lettuce cultivars resistant to fusarium would be the solution to the problem.